/*
    * Copyright 2019 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   https://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package io.netty.handler.codec.http2;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.channel.Channel;
  import io.netty.channel.ChannelConfig;
  import io.netty.channel.ChannelFuture;
  import io.netty.channel.ChannelFutureListener;
  import io.netty.channel.ChannelHandler;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelPipeline;
  import io.netty.channel.EventLoop;
  import io.netty.channel.ServerChannel;
  import io.netty.channel.socket.ChannelInputShutdownReadComplete;
  import io.netty.channel.socket.ChannelOutputShutdownEvent;
  import io.netty.handler.codec.http2.Http2FrameCodec.DefaultHttp2FrameStream;
  import io.netty.handler.ssl.SslCloseCompletionEvent;
  import io.netty.util.ReferenceCounted;
  import io.netty.util.internal.ObjectUtil;
  
  import java.util.ArrayDeque;
  import java.util.ArrayList;
  import java.util.HashSet;
  import java.util.Queue;
  import java.util.Set;
  import javax.net.ssl.SSLException;
  
  import static io.netty.handler.codec.http2.AbstractHttp2StreamChannel.CHANNEL_INPUT_SHUTDOWN_READ_COMPLETE_VISITOR;
  import static io.netty.handler.codec.http2.AbstractHttp2StreamChannel.CHANNEL_OUTPUT_SHUTDOWN_EVENT_VISITOR;
  import static io.netty.handler.codec.http2.AbstractHttp2StreamChannel.SSL_CLOSE_COMPLETION_EVENT_VISITOR;
  import static io.netty.handler.codec.http2.Http2Error.INTERNAL_ERROR;
  import static io.netty.handler.codec.http2.Http2Exception.connectionError;
  
  /**
   * An HTTP/2 handler that creates child channels for each stream. This handler must be used in combination
   * with {@link Http2FrameCodec}.
   *
   * <p>When a new stream is created, a new {@link Http2StreamChannel} is created for it. Applications send and
   * receive {@link Http2StreamFrame}s on the created channel. {@link ByteBuf}s cannot be processed by the channel;
   * all writes that reach the head of the pipeline must be an instance of {@link Http2StreamFrame}. Writes that reach
   * the head of the pipeline are processed directly by this handler and cannot be intercepted.
   *
   * <p>The child channel will be notified of user events that impact the stream, such as {@link
   * Http2GoAwayFrame} and {@link Http2ResetFrame}, as soon as they occur. Although {@code
   * Http2GoAwayFrame} and {@code Http2ResetFrame} signify that the remote is ignoring further
   * communication, closing of the channel is delayed until any inbound queue is drained with {@link
   * Channel#read()}, which follows the default behavior of channels in Netty. Applications are
   * free to close the channel in response to such events if they don't have use for any queued
   * messages. Any connection level events like {@link Http2SettingsFrame} and {@link Http2GoAwayFrame}
   * will be processed internally and also propagated down the pipeline for other handlers to act on.
   *
   * <p>Outbound streams are supported via the {@link Http2StreamChannelBootstrap}.
   *
   * <p>{@link ChannelConfig#setMaxMessagesPerRead(int)} and {@link ChannelConfig#setAutoRead(boolean)} are supported.
   *
   * <h3>Reference Counting</h3>
   *
   * Some {@link Http2StreamFrame}s implement the {@link ReferenceCounted} interface, as they carry
   * reference counted objects (e.g. {@link ByteBuf}s). The multiplex codec will call {@link ReferenceCounted#retain()}
   * before propagating a reference counted object through the pipeline, and thus an application handler needs to release
   * such an object after having consumed it. For more information on reference counting take a look at
   * <a href="https://netty.io/wiki/reference-counted-objects.html">the reference counted docs.</a>
   *
   * <h3>Channel Events</h3>
   *
   * A child channel becomes active as soon as it is registered to an {@link EventLoop}. Therefore, an active channel
   * does not map to an active HTTP/2 stream immediately. Only once a {@link Http2HeadersFrame} has been successfully sent
   * or received, does the channel map to an active HTTP/2 stream. In case it is not possible to open a new HTTP/2 stream
   * (i.e. due to the maximum number of active streams being exceeded), the child channel receives an exception
   * indicating the cause and is closed immediately thereafter.
   *
   * <h3>Writability and Flow Control</h3>
   *
   * A child channel observes outbound/remote flow control via the channel's writability. A channel only becomes writable
   * when it maps to an active HTTP/2 stream . A child channel does not know about the connection-level flow control
   * window. {@link ChannelHandler}s are free to ignore the channel's writability, in which case the excessive writes will
   * be buffered by the parent channel. It's important to note that only {@link Http2DataFrame}s are subject to
   * HTTP/2 flow control.
   *
   * <h3>Closing a {@link Http2StreamChannel}</h3>
   *
   * Once you close a {@link Http2StreamChannel} a {@link Http2ResetFrame} will be sent to the remote peer with
   * {@link Http2Error#CANCEL} if needed. If you want to close the stream with another {@link Http2Error} (due
   * errors / limits) you should propagate a {@link Http2FrameStreamException} through the {@link ChannelPipeline}.
   * Once it reaches the end of the {@link ChannelPipeline} it will automatically close the {@link Http2StreamChannel}
  * and send a {@link Http2ResetFrame} with the unwrapped {@link Http2Error} set. Another possibility is to just
  * directly write a {@link Http2ResetFrame} to the {@link Http2StreamChannel}l.
  */
 public final class Http2MultiplexHandler extends Http2ChannelDuplexHandler {
 
     static final ChannelFutureListener CHILD_CHANNEL_REGISTRATION_LISTENER = Http2MultiplexHandler::registerDone;
 
     private final ChannelHandler inboundStreamHandler;
     private final ChannelHandler upgradeStreamHandler;
     private final Queue<AbstractHttp2StreamChannel> readCompletePendingQueue =
             new MaxCapacityQueue<AbstractHttp2StreamChannel>(new ArrayDeque<AbstractHttp2StreamChannel>(8),
                     // Choose 100 which is what is used most of the times as default.
                     Http2CodecUtil.SMALLEST_MAX_CONCURRENT_STREAMS);
 
     // Outbound child channels opened via Http2StreamChannelBootstrap that were not closed yet. Until their first
     // HEADERS frame is written their stream is unknown to the connection, so closing the connection doesn't close them.
     private final Set<AbstractHttp2StreamChannel> outboundStreamChannels = new HashSet<AbstractHttp2StreamChannel>();
     // Shared by all outbound child channels: removes a child channel from outboundStreamChannels once it is closed.
     private final ChannelFutureListener outboundStreamChannelCloseListener =
             future -> outboundStreamChannels.remove(future.channel());
     private boolean parentReadInProgress;
     private int idCount;
 
     // Need to be volatile as accessed from within the Http2MultiplexHandlerStreamChannel in a multi-threaded fashion.
     private volatile ChannelHandlerContext ctx;
 
     /**
      * Creates a new instance
      *
      * @param inboundStreamHandler the {@link ChannelHandler} that will be added to the {@link ChannelPipeline} of
      *                             the {@link Channel}s created for new inbound streams.
      */
     public Http2MultiplexHandler(ChannelHandler inboundStreamHandler) {
         this(inboundStreamHandler, null);
     }
 
     /**
      * Creates a new instance
      *
      * @param inboundStreamHandler the {@link ChannelHandler} that will be added to the {@link ChannelPipeline} of
      *                             the {@link Channel}s created for new inbound streams.
      * @param upgradeStreamHandler the {@link ChannelHandler} that will be added to the {@link ChannelPipeline} of the
      *                             upgraded {@link Channel}.
      */
     public Http2MultiplexHandler(ChannelHandler inboundStreamHandler, ChannelHandler upgradeStreamHandler) {
         this.inboundStreamHandler = ObjectUtil.checkNotNull(inboundStreamHandler, "inboundStreamHandler");
         this.upgradeStreamHandler = upgradeStreamHandler;
     }
 
     static void registerDone(ChannelFuture future) {
         // Handle any errors that occurred on the local thread while registering. Even though
         // failures can happen after this point, they will be handled by the channel by closing the
         // childChannel.
         if (!future.isSuccess()) {
             Channel childChannel = future.channel();
             if (childChannel.isRegistered()) {
                 childChannel.close();
             } else {
                 childChannel.unsafe().closeForcibly();
             }
         }
     }
 
     @Override
     protected void handlerAdded0(ChannelHandlerContext ctx) {
         if (ctx.executor() != ctx.channel().eventLoop()) {
             throw new IllegalStateException("EventExecutor must be EventLoop of Channel");
         }
         this.ctx = ctx;
     }
 
     @Override
     protected void handlerRemoved0(ChannelHandlerContext ctx) {
         readCompletePendingQueue.clear();
     }
 
     @Override
     public void channelInactive(ChannelHandlerContext ctx) throws Exception {
         if (!outboundStreamChannels.isEmpty()) {
             // The streams known to the connection were closed already. Close the outbound child channels whose
             // stream was never created as well, so they see channelInactive() and their closeFuture() completes.
             // Iterate over a copy, as closing a child channel removes it from outboundStreamChannels (see
             // outboundStreamChannelCloseListener).
             for (AbstractHttp2StreamChannel childChannel :
                     new ArrayList<AbstractHttp2StreamChannel>(outboundStreamChannels)) {
                 if (childChannel.stream().state() == Http2Stream.State.IDLE) {
                     childChannel.unsafe().closeForcibly();
                 }
             }
         }
         ctx.fireChannelInactive();
     }
 
     @Override
     public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
         parentReadInProgress = true;
         if (msg instanceof Http2StreamFrame) {
             if (msg instanceof Http2WindowUpdateFrame) {
                 // We dont want to propagate update frames to the user
                 return;
             }
             Http2StreamFrame streamFrame = (Http2StreamFrame) msg;
             DefaultHttp2FrameStream s =
                     (DefaultHttp2FrameStream) streamFrame.stream();
 
             AbstractHttp2StreamChannel channel = (AbstractHttp2StreamChannel) s.attachment;
             if (msg instanceof Http2ResetFrame || msg instanceof Http2PriorityFrame) {
                 // Reset and Priority frames needs to be propagated via user events as these are not flow-controlled and
                 // so must not be controlled by suppressing channel.read() on the child channel.
                 channel.fireChildUserEventTriggered(msg);
 
                 // RST frames will also trigger closing of the streams which then will call
                 // AbstractHttp2StreamChannel.streamClosed()
             } else {
                 channel.fireChildRead(streamFrame);
             }
             return;
         }
 
         if (msg instanceof Http2GoAwayFrame) {
             // goaway frames will also trigger closing of the streams which then will call
             // AbstractHttp2StreamChannel.streamClosed()
             onHttp2GoAwayFrame(ctx, (Http2GoAwayFrame) msg);
         }
 
         // Send everything down the pipeline
         ctx.fireChannelRead(msg);
     }
 
     @Override
     public void channelWritabilityChanged(final ChannelHandlerContext ctx) throws Exception {
         if (ctx.channel().isWritable()) {
             // While the writability state may change during iterating of the streams we just set all of the streams
             // to writable to not affect fairness. These will be "limited" by their own watermarks in any case.
             forEachActiveStream(AbstractHttp2StreamChannel.WRITABLE_VISITOR);
         }
 
         ctx.fireChannelWritabilityChanged();
     }
 
     @Override
     public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
         if (evt instanceof Http2FrameStreamEvent) {
             Http2FrameStreamEvent event = (Http2FrameStreamEvent) evt;
             DefaultHttp2FrameStream stream = (DefaultHttp2FrameStream) event.stream();
             if (event.type() == Http2FrameStreamEvent.Type.State) {
                 switch (stream.state()) {
                     case HALF_CLOSED_LOCAL:
                         if (stream.id() != Http2CodecUtil.HTTP_UPGRADE_STREAM_ID) {
                             // Ignore everything which was not caused by an upgrade
                             break;
                         }
                         // fall-through
                     case HALF_CLOSED_REMOTE:
                         // fall-through
                     case OPEN:
                         if (stream.attachment != null) {
                             // ignore if child channel was already created.
                             break;
                         }
                         final AbstractHttp2StreamChannel ch;
                         // We need to handle upgrades special when on the client side.
                         if (stream.id() == Http2CodecUtil.HTTP_UPGRADE_STREAM_ID && !isServer(ctx)) {
                             // We must have an upgrade handler or else we can't handle the stream
                             if (upgradeStreamHandler == null) {
                                 throw connectionError(INTERNAL_ERROR,
                                         "Client is misconfigured for upgrade requests");
                             }
                             ch = new Http2MultiplexHandlerStreamChannel(stream, upgradeStreamHandler);
                             ch.closeOutbound();
                         } else {
                             ch = new Http2MultiplexHandlerStreamChannel(stream, inboundStreamHandler);
                         }
                         ChannelFuture future = ctx.channel().eventLoop().register(ch);
                         if (future.isDone()) {
                             registerDone(future);
                         } else {
                             future.addListener(CHILD_CHANNEL_REGISTRATION_LISTENER);
                         }
                         break;
                     case CLOSED:
                         AbstractHttp2StreamChannel channel = (AbstractHttp2StreamChannel) stream.attachment;
                         if (channel != null) {
                             channel.streamClosed();
                         }
                         break;
                     default:
                         // ignore for now
                         break;
                 }
             }
             return;
         }
         if (evt == ChannelInputShutdownReadComplete.INSTANCE) {
             forEachActiveStream(CHANNEL_INPUT_SHUTDOWN_READ_COMPLETE_VISITOR);
         } else if (evt == ChannelOutputShutdownEvent.INSTANCE) {
             forEachActiveStream(CHANNEL_OUTPUT_SHUTDOWN_EVENT_VISITOR);
         } else if (evt == SslCloseCompletionEvent.SUCCESS) {
             forEachActiveStream(SSL_CLOSE_COMPLETION_EVENT_VISITOR);
         }
         ctx.fireUserEventTriggered(evt);
     }
 
     // TODO: This is most likely not the best way to expose this, need to think more about it.
     Http2StreamChannel newOutboundStream() {
         final Http2MultiplexHandlerStreamChannel childChannel =
                 new Http2MultiplexHandlerStreamChannel((DefaultHttp2FrameStream) newStream(), null);
         outboundStreamChannels.add(childChannel);
         childChannel.closeFuture().addListener(outboundStreamChannelCloseListener);
         return childChannel;
     }
 
     @Override
     public void exceptionCaught(ChannelHandlerContext ctx, final Throwable cause) throws Exception {
         if (cause instanceof Http2FrameStreamException) {
             Http2FrameStreamException exception = (Http2FrameStreamException) cause;
             Http2FrameStream stream = exception.stream();
             AbstractHttp2StreamChannel childChannel = (AbstractHttp2StreamChannel)
                     ((DefaultHttp2FrameStream) stream).attachment;
             try {
                 childChannel.fireChildExceptionCaught(cause.getCause());
             } finally {
                 // Close with the correct error that causes this stream exception.
                 // See https://github.com/netty/netty/issues/13235#issuecomment-1441994672
                 childChannel.closeWithError(exception.error());
             }
             return;
         }
         if (cause instanceof Http2MultiplexActiveStreamsException) {
             // Unwrap the cause that was used to create it and fire it for all the active streams.
             fireExceptionCaughtForActiveStream(cause.getCause());
             return;
         }
 
         if (cause.getCause() instanceof SSLException) {
             fireExceptionCaughtForActiveStream(cause);
         }
         ctx.fireExceptionCaught(cause);
     }
 
     private void fireExceptionCaughtForActiveStream(final Throwable cause) throws Http2Exception {
         forEachActiveStream(new Http2FrameStreamVisitor() {
             @Override
             public boolean visit(Http2FrameStream stream) {
                 AbstractHttp2StreamChannel childChannel = (AbstractHttp2StreamChannel)
                         ((DefaultHttp2FrameStream) stream).attachment;
                 childChannel.fireChildExceptionCaught(cause);
                 return true;
             }
         });
     }
 
     private static boolean isServer(ChannelHandlerContext ctx) {
         return ctx.channel().parent() instanceof ServerChannel;
     }
 
     private void onHttp2GoAwayFrame(ChannelHandlerContext ctx, final Http2GoAwayFrame goAwayFrame) {
         if (goAwayFrame.lastStreamId() == Integer.MAX_VALUE) {
             // None of the streams can have an id greater than Integer.MAX_VALUE
             return;
         }
         // Notify which streams were not processed by the remote peer and are safe to retry on another connection:
         try {
             final boolean server = isServer(ctx);
             forEachActiveStream(new Http2FrameStreamVisitor() {
                 @Override
                 public boolean visit(Http2FrameStream stream) {
                     final int streamId = stream.id();
                     if (streamId > goAwayFrame.lastStreamId() && Http2CodecUtil.isStreamIdValid(streamId, server)) {
                         final AbstractHttp2StreamChannel childChannel = (AbstractHttp2StreamChannel)
                                 ((DefaultHttp2FrameStream) stream).attachment;
                         childChannel.fireChildUserEventTriggered(goAwayFrame.retainedDuplicate());
                     }
                     return true;
                 }
             });
         } catch (Http2Exception e) {
             ctx.fireExceptionCaught(e);
             ctx.close();
         }
     }
 
     /**
      * Notifies any child streams of the read completion.
      */
     @Override
     public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
         processPendingReadCompleteQueue();
         ctx.fireChannelReadComplete();
     }
 
     private void processPendingReadCompleteQueue() {
         parentReadInProgress = true;
         // If we have many child channel we can optimize for the case when multiple call flush() in
         // channelReadComplete(...) callbacks and only do it once as otherwise we will end-up with multiple
         // write calls on the socket which is expensive.
         AbstractHttp2StreamChannel childChannel = readCompletePendingQueue.poll();
         if (childChannel != null) {
             try {
                 do {
                     childChannel.fireChildReadComplete();
                     childChannel = readCompletePendingQueue.poll();
                 } while (childChannel != null);
             } finally {
                 parentReadInProgress = false;
                 readCompletePendingQueue.clear();
                 ctx.flush();
             }
         } else {
             parentReadInProgress = false;
         }
     }
 
     private final class Http2MultiplexHandlerStreamChannel extends AbstractHttp2StreamChannel {
 
         Http2MultiplexHandlerStreamChannel(DefaultHttp2FrameStream stream, ChannelHandler inboundHandler) {
             super(stream, ++idCount, inboundHandler);
         }
 
         @Override
         protected boolean isParentReadInProgress() {
             return parentReadInProgress;
         }
 
         @Override
         protected void addChannelToReadCompletePendingQueue() {
             // If there is no space left in the queue, just keep on processing everything that is already
             // stored there and try again.
             while (!readCompletePendingQueue.offer(this)) {
                 processPendingReadCompleteQueue();
             }
         }
 
         @Override
         protected ChannelHandlerContext parentContext() {
             return ctx;
         }
     }
 }